Angular rate of rotation about a given coordinate axis may be measured by moving (e.g., vibrating) an accelerometer along an axis normal to the accelerometer's sensitive axis and normal to the rate axis about which rotation is to be measured. For example, consider a set of X, Y, Z, coordinate axes fixed in a body whose rotation rate is to be measured, and an accelerometer also fixed in the body with its sensitive axis aligned along the Z axis. If the angular rotation vector of the body includes a component along the X axis, then periodic motion of the accelerometer along the Y axis will result in a periodic Coriolis acceleration acting in the Z direction that will be sensed by the accelerometer. The magnitude of the Coriolis acceleration is proportional to the velocity along the Y axis and the rotation rate about the X axis. As a result, the output of the accelerometer includes a DC or slowly changing component that represents the linear acceleration of the body along the Z axis, and a periodic component that represents the rotation of the body about the X axis. The accelerometer output can be processed, along with the outputs of accelerometers that have their sensitive axes in the X and Y directions and that are moved along the Z and X axes, respectively, to yield linear acceleration and angular rate about the X, Y and Z axes. Such signal processing is described in U.S. Pat. Nos. 4,445,376 and 4,590,801.
As described in U.S. Pat. No. 4,590,801, one preferred embodiment of a rotation rate sensor comprises, for each axis, two accelerometers oriented with their sensitive axes parallel or antiparallel to one another, and means for dithering (i.e., vibrating) the accelerometers along an axis normal to their sensitive axes. A suitable method for mounting such accelerometer pairs is described in U.S. Pat. No. 4,510,802. A side view of a structure shown in FIG. 3 of that patent illustrates a mount for two accelerometers centered on mounting surfaces at the top and at the bottom of a parallelogram frame. The side of the parallelogram frame shown in the figure includes six pivots, comprising thin metal flexures aligned with their bending axes in parallel, one of the flexures being disposed at each of the four corners and at the centers of the vertical sides of the parallelogram frame.
Prior designs for rotation rate sensors like the one briefly described above have been subject to problems associated with dynamic imbalance of the parallelogram structure, and the sensitivity of the structure to vibration occurring in a direction other than parallel to the axis along which they are dithered. For example, if the parallelogram frame just described is subjected to a vibration along an axis parallel with the sensitive axes of the accelerometers, both accelerometers should experience a common mode force, and produce an equal, though opposite, output that cancels in the angular rate channel. However, due to the transverse (i.e., cross axis) flexibility of the thin metal flexures and of the supporting surfaces on which the accelerometers are mounted, the same vibrational movement is not applied to the two accelerometers. Any structural resonance in the frame may exacerbate this problem. As a result, the output signal produced by the rate sensor includes an error in the angular rate channel that is proportional to the difference in the nominal common mode vibration to which the accelerometers are subjected.
The prior art driving mechanism used to provide the dither motion or vibration along an axis transverse to the sensitive axes of the accelerometers is typically disposed adjacent one side of the parallelogram frame. Due to an inequality of mass distribution (or moment of inertia) between the driving mechanism and the parallelogram frame, the parallelogram frame may experience an imbalance torque when both the frame and driving mechanism are subjected to a vibration directed parallel to the accelerometer sensitive axes. In addition differences in the moment of inertia of the driving mechanism and the parallelogram frame may cause a variation in the driving force applied to the frame, when it is subjected to an angular vibration about its rotation sensitive axis. Either of these two effects may result in a substantial error in the angular rate channel signal.